By [[http://tvtropes.org/pmwiki/pmwiki.php/Tropers/ScanVisor ScanVisor]]

To make this short, [[TitleDrop Hell is not other people,]] because Hell is people like yourself. In other words, hell is yourself.

The fact that everyone refuses to leave despite the door being unlocked is basically a direct statement on a person's need for other people, regardless of who they are.

The unlocked door itself symbolizes the potential for change, while the people like ourselves in the room symbolize the reflections of our negative traits we see in others, whilst the two symbols combined are themselves a symbol of the human potential to escape from one's self, from one's already defined qualities, into something different, and possibly better.

In short: the room is you. The other people in the room reflect your negative traits that you see in others, and you within the room represents your tendency to self-aggrandize and cast your worse parts onto others, as opposed to taking responsibility for them yourself. The unlocked door represents the idea that "no man is an island."

Plus, [[FridgeLogic how could a lifelong leftist, someone who believes almost primarily in unity, say that hell is other people?]]